Tortoise shell: Best. Expensive. Illegal. Plastic: Adequate. Cheap. Legal. Buffalo horn: Nearly as good as tortoise-shell. Less expensive. Legal. Quarters: An acquired taste, and mostly for bluegrass mandolin. Cost $0.25. Legal (tender) Coffee-can lid: Only for dulcimer, and then only for strummed, rather than plucked dulcimer. Free (though you have to like coffee). Legal, but who cares?